The radiolysis of thin water ice in electron microscopy

Little is known about the radiolysis of water ice, especially as compared to the radiolysis of liquid water. In this study, the radiolytic decomposition of thin water ice films is probed for an initial 80 keV electron beam using electron energy loss spectroscopy, EELS. Pre-peaks in the oxygen K-edge spectra give the relative intensities of most of the oxygen containing species produced by radiolytic water decomposition. Contrary to expectations from the extrapolation of liquid water radiolysis data to high dose rates where significant H2O2 production is expected, the main molecular product observed in the oxygen K-edge EELS spectra of water ice is O2. Significant mass loss of water and decomposition of most of its radiolytic products are observed for higher exposures, and the O atom seems to have a major role in the subsequent chemistry. An inverse relationship between the formation of O2 and of ·OH is observed. A new high-dose reaction scheme is proposed. The significance of our results with respect to the radiolysis of water ice and for electron microscopy studies is discussed.